1. Field of Invention
This invention relates to a process for the separation of catalyst from hydrocarbon in a fluid catalytic cracking unit (FCU).
2. Background
Gasoline and distillate liquid hydrocarbon fuels are the primary finished products for most petroleum refiners. These fuels boil in the range of about 100.degree. F. to about 650.degree. F. However, the crude oil from which these fuels are derived can often contain from 30 to 70 percent by volume hydrocarbon boiling above 650.degree. F. The process of fluid catalytic cracking breaks apart high boiling point, high molecular weight molecules into lower boiling point, lower molecular weight products that can be blended into gasoline and distillate fuels.
Fluid catalytic cracking units operate through the introduction of a hot fluidized catalytic cracking catalyst into a high molecular weight feed at the upstream end of a riser reactor. Once contacted with the hot catalyst, the feed is vaporized, carrying a suspension of catalyst and hydrocarbon up through the riser reactor. The hot catalyst supplies all or a major portion of the heat necessary to vaporize the hydrocarbon feed and carry out the endothermic catalytic cracking reaction.
The suspension of catalyst and hydrocarbon vapor passes up the riser reactor at high velocity. However, due to the high activity of the catalyst, the cracking reaction generally proceeds to the desired extent prior to or upon reaching the upper or downstream end of the riser reactor. The cracked hydrocarbon must then be separated from the catalyst and further processed into upgraded products. The catalyst, which has accumulated coke in the cracking reaction, must be stripped to remove extraneous hydrocarbons and regenerated prior to reintroduction into the riser reactor. Process improvements in this separation and stripping step is the subject of this invention.
Many catalytic cracking advancements have been made in the area of catalyst separation, catalyst stripping, and prevention of undesired catalytic reactions. Some catalytic cracking equipment had bed crackers with sloped risers. The sloped riser performed the function of carrying the oil and catalyst to the catalyst bed where most of the reaction occurred. Slower catalytic reaction times facilitated the operation of bed crackers and were a result of the lower activity catalyst prevalent at the time and lower reaction temperatures. Catalyst separation from hydrocarbon was performed in cyclones erected in the reaction vessel. Quick disengaging of catalyst from hydrocarbon was not as necessary to prevent undesired overcracking reactions due to the lower catalyst activity and reaction temperatures. Catalyst stripping was performed in a stripper section communicating with the catalyst bed.
As crude costs increased, gasoline volume and octane requirements remained strong, and the phase out of lead from gasoline took effect, refiners stepped up cracking catalyst development efforts. High activity catalysts, particularly crystalline zeolite cracking catalysts, were developed, followed by processing techniques and equipment permitting higher reactor temperatures. However, as reaction temperatures and catalyst to oil ratio were increased, it was observed that much of the desired catalytic reaction was occurring in the riser. Refiners began developing processes designed to perform the cracking reactions in the riser. The fundamental process change required longer, more vertically positioned riser reactors, which resulted in more effective catalyst to oil mixing. The vertical riser facilities reduced undesirable light gas production, increased feed conversion to light products, increased gasoline octane, and lowered undesirable coke production.
An unexpected penalty associated with higher catalyst activity and higher reactor temperatures was the occurrence of catalytic overcracking and thermal cracking. Unless the catalyst was quickly removed from the hydrocarbon, undesirable overcracking reactions would occur, reducing gasoline yield and increasing light gas production. Older prior art catalytic cracking units were not equipped to mitigate this condition. Newer facilities began to recognize the problems associated with overcracking and thermal cracking and included roughcut cyclone separation erected in close proximity to or communicating with the riser reactor to help reduce the problem.
In some types of catalytic cracking units, the riser penetrates the center of the disengager vessel. These units afford quick separation of catalyst from oil by positioning an inverted can over the riser outlet. The catalyst and hydrocarbon is directed downwards where the catalyst is directed towards a stripping section positioned immediately below the disengaging section of the disengager vessel or to a separate stripper vessel. The hydrocarbon pressures back through the inverted can and is further separated from catalyst in secondary cyclones prior to exiting the disengager. The extended hydrocarbon flow pattern between the inverted can and the secondary cyclones permits undesirable thermal cracking reactions to occur at high reaction temperatures and detracts from the utility of center riser designs.
The center riser facility also can have a completely enclosed internal "hot-wall" roughcut separator and secondary cyclones. Enclosed "hot-wall" roughcut separator designs translate into more costly and time-consuming maintenance. Prior art internal "hot-wall" vessels require more expensive metallurgy, thicker steel, exotic refractory and erosion protection, as well as more costly rigging to assemble the cyclone within another vessel than the external cyclone alternative. Moreover, internal cyclone failures in hot-wall vessels are difficult to visually detect. Repairs are also more difficult to perform, usually requiring unit shutdown as well as long, time-consuming preparation steps prior to and upon entry into the disengager vessel.
Some prior art catalytic cracking units have an external positioned vertical riser with a closely connected external roughcut separator. Such units provide quick separation of catalyst from oil by the close proximity of the roughcut separator to the riser outlet. However, the process is more expensive to build due to additional ductwork and plot space requirement.
Other prior art catalytic cracking units have been employed to address many of the objectives and problems noted above, each with varying degrees of success and limitations.
Anderson et al., U.S. Pat. No. 4,043,899, describes internal cyclones which have been modified to include cyclonic stripping of catalyst separated from hydrocarbon vapors from a center riser catalytic cracking unit.
Parker et al., U.S. Pat. No. 4,455,220, describes a single vessel cyclone separator and stripper process having a vortex stabilizer mechanism separating the two vessel sections. The Parker design also has a secondary cyclone connected directly to the single vessel roughcut cyclone outlet without benefit of a disengaging space. While the design features less equipment and can be built for a lower cost, the generically nonuniform flow of riser reactors can pose difficulties for these systems. When the riser outlet flow surges upwards, roughcut separation efficiency is greatly reduced and excessive amounts of hydrocarbon can drop down to the stripper section while excessive amounts of catalyst spew out the top of the cyclone. This continuous cycling results in undesired overcracking in the roughcut cyclone hydrocarbon outlet and the potential for catalyst defeating the secondary cyclone and breaking through to downstream equipment.